Hereinafter, a “Q” or “q” prefix in a word or phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A superconducting qubit may include a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a superconducting qubit, the Josephson junction—which has an inductance—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear resonator.
The information processed by qubits is emitted in the form of microwave energy in a range of microwave frequencies. The microwave emissions are captured, processed, and analyzed to decipher the quantum information encoded therein. For quantum computing of qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other types of superconducting quantum logic circuits, must not alter the energy states of the particles or the microwave emissions in any significant manner. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and/or superconductor structures that are used in such a circuit.
The readout circuitry is generally coupled with a qubit by electromagnetic resonance (usually a microwave or radio-frequency resonance) using a resonator. A resonator in the readout circuitry comprises inductive and capacitive elements. The illustrative embodiments recognize that a superconducting capacitive coupling used with a superconducting quantum logic circuit, and particularly to couple a readout circuit with a qubit is significantly larger in size than the size of the Josephson junction therein. FIG. 1 depicts a scaled image of qubit with a presently fabricated capacitive coupling to external circuits. Image 100 shows a portion of a qubit chip. Coupling capacitors 102 couple with transmission lines (not visible) that bring the electromagnetic signal out from Josephson junction 104. Capacitor pads 106 are capacitive devices driving Josephson junction 104 and forming a nonlinear resonator. A ground-plane (not visible) typically surrounds all or a portion of this structure.
As can be seen, fabricating capacitive coupling structures 102 in a coplanar manner with the structures of qubit 100 takes up the very limited planar real-estate on the fabrication plane of chip 100. Josephson junction 104—which is barely visible in the image of this figure—occupies only a fraction of the exaggerated box drawn around the junction to identify its position. The area occupied by capacitive coupling structures 102 is significantly more than the area of Josephson junction 104.
A capacitive coupling structure as in any one of the capacitive coupling structures 102, is fabricated coplanar with the qubit elements such as the Josephson junction and the junction's driving capacitors. The illustrative embodiments recognize that fabricating capacitive coupling devices as coplanar to the qubit circuit elements limits the number of qubits that can be fabricated per die in a fabrication process. The illustrative embodiments recognize that a need exists for a method of fabricating a capacitive coupling device that is not in the same plane of fabrication as the Josephson junction or its driving capacitors.
A capacitive coupling structure that can be used in place of any one of the capacitive coupling structures 102, is interchangeably referred to herein as C-coupler. A superconducting C-coupler according to an illustrative embodiment is not coplanar with the qubit elements. A plane of a fabrication substrate, e.g., a silicon substrate of a semiconducting wafer, on which the superconducting qubit elements are fabricated is referred to herein as a “front” side (front, frontside) regardless of the actual orientation of the plane during fabrication. A “back” side (back, backside) of the substrate is opposite the frontside, to wit, an opposite surface of the same wafer which is substantially parallel to the front side of the wafer.
The structures of a superconducting C-coupler are fabricated from the backside, through the substrate, in a substantially perpendicular direction from the frontside plane of fabrication of the qubit. A structure that is formed through a silicon substrate in a direction perpendicular to a plane of fabrication is referred to as a “Through-Silicon via” or “TSV” or simply a “via”. Normally, a via passes completely through the silicon substrate from one side—e.g. the frontside—to the other side—e.g., the backside. A structure of the superconducting C-coupler protrudes partially through the thickness of the substrate between the frontside and the backside. Such a structure is referred to herein as a “partial via”.
This manner of fabricating a superconducting C-coupler allows the capacitive coupling to be removed from the fabrication plane of the qubit, freeing up space in that plane for more qubit elements but still enabling the capacitive coupling between qubit elements and a readout circuit. Additionally, the partial vias of the superconducting C-coupler allow the readout circuitry to also be desirably placed or fabricated on the back side.